Improving Relay Protection Levels in Medium-Voltage Switchgear

This paper demonstrates the impacts of an electronically triggered fault current limiter (ET-FCL) on typical single-ended and main-tie-main (MTM) medium-voltage switchgear arrangements. The fast acting (less than 1 ms) ET-FCL described in this paper effectively opens the normally closed tie connection between two mains, thereby disconnecting one of the sources to the fault. An ultrafast opening of the tie connection simplifies protection coordination by not having to coordinate the main breaker relay with the tie breaker relay at maximum fault levels. System benefits include the following: significantly lower arc flash energy for downstream faults, faster backup clearing times for a feeder relay/breaker failure, reduction of motor starter dropouts due to fault-produced voltage dips, improved system stability, improvement in utility's CAIDI reliability index, reduced stress on switchgear breakers, and overall lower capital costs on new construction (switchgear, cables, etc.). This paper demonstrates relay setting changes on typical single-ended and MTM arrangement with and without applying the ET-FCL. Impacts on incident energy reduction, backup clearing times, dynamic system stability, and voltage dips will be discussed.